Computer systems configured as local area networks have been common for nearly two decades and are popular in a wide variety of business and educational applications. The most common LANs comprise a number of processing devices and a server that are coupled together by a hard-wired connection. Since about 1990, however, wireless local area networks (LANs) have become more common in the marketplace. Although the concept behind wireless LANS had been described a decade earlier, interest in LAN networks was limited until the release of the 2.4 GHz unlicensed band for industrial, scientific and medical (ISM) applications. Wireless LAN products most often employ either direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS) techniques to communicate between roaming mobile stations and network access points.
In a typical wireless computer network environment, the "backbone" of the LAN is a central server that communicates with a number of network access points through a hard-wired connection. Each access point (AP) includes a transceiver for communicating with at least one roaming mobile station (MS). The mobile station may be a point-of-sale terminal (i.e., an electronic cash register), a bar code reader or other scanner device, or a notepad, desktop or laptop computer. Each MS establishes a communication link with an AP by scanning the ISM band to find an available AP. Once a reliable link is established, the MS interacts with other mobile stations and/or the server. This allows the user of the MS to move freely in the office, factory, hospital or other facility where the wireless LAN is based, without being limited by the length of a hard-wired connection to the LAN.
Eventually, the mobile station will move out of the range of its current access point. When this occurs, a "handover" takes place that breaks down the communication link between the mobile station and the current access point and establishes a new communication link between the mobile station and a new access point. The mobile station initiates this process when it detects that the link quality with the current access point has degraded below a specified threshold. The mobile station then begins looking for another access point, probably in a different frequency channel.
As noted, wireless LAN products frequently employ some type of spread spectrum technique, such as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS), to communicate between roaming mobile stations and network access points. A distinguishing feature of the spread spectrum technique is that the modulated output signals occupy a much greater transmission bandwidth than the baseband information bandwidth requires. The spreading is achieved by encoding each data bit in the baseband information using a codeword, or symbol, that has a much higher frequency than the baseband information bit rate. The resultant "spreading" of the signal across a wider frequency bandwidth results in comparatively lower power spectral density, so that other communication systems are less likely to suffer interference from the device that transmits the spread spectrum signal. It also makes the spread signal harder to detect and less susceptible to interference (i.e., harder to jam).
Both DSSS and FHSS techniques employ a pseudo-random (PN) codeword known to the transmitter and to the receiver to spread the data and to make it more difficult to detect by receivers lacking the codeword. The codeword consists of a sequence of "chips" having values of -1 or +1 (polar) or 0 and 1 (non-polar) that are multiplied by (or Exclusive-ORed with) the information bits to be transmitted. Accordingly, a logic "0" information bit may be encoded as a non-inverted codeword sequence, and a logic "1" information bit may be encoded as an inverted codeword sequence. Alternatively, a logic "0" information bit may be encoded as a first predetermined codeword sequence and a logic "1" information bit may be encoded as a second predetermined codeword sequence. There are numerous well known codes, including M-sequences, Gold codes and Kasami codes.
Many wireless networks conform to the IEEE 802.11 standard, which employs the well-know Barker code to encode and spread the data. The Barker codeword consists of eleven chips having the sequence "00011101101", or "+++---+--+-". One entire Barker codeword sequence, or symbol, is transmitted in the time period occupied by a single binary information bit. Thus, if the symbol (or Barker sequence) rate is 1 MHZ, the underlying chip rate for the eleven chips in the sequence is 11 MHZ. By using the 11 MHZ chip rate signal to modulate the carrier wave, the spectrum occupied by the transmitted signal is eleven times greater. Accordingly, the recovered signal in the receiver, after demodulation and correlation, comprises a series of inverted Barker sequences representing, for example, logic "1" information bits, and non-inverted Barker sequences, representing for example, logic "0" information bits.
A key performance parameter of any communication system, particularly computer networks and cellular telephone systems, and the like, is the transfer rate of data between devices in the communication system. Wireless LANs are no exception. It is therefore important to maximize the rate at which data may be exchanged between access points and mobile stations in a wireless LAN in order to maximize the LAN performance.
Accordingly, there is a need in the art for systems and methods that increase the rate at which data may be transferred in a communication system using spread spectrum techniques to communicate data between a receiver and a transmitter. There is a still further need for systems and methods that increase the rate at which data may be transferred in a wireless LAN using spread spectrum techniques to communicate data between a network access point and a mobile station in the network.